Project Summary Medulloblastomas are the most common malignant pediatric tumor of the central nervous system. The current treatment paradigm for medulloblastomas includes surgical resection, irradiation of the tumor site and craniospinal irradiation as well as chemotherapy. While this regimen is associated with a ~70% cure rate, survivors are beset with long-term neurological side effects, and tumor recurrence is fatal. Thus, there is a need to develop therapeutic approaches that reduce the requirement for irradiation, reduce the incidence of recurrence, and may be successfully applied to patients that relapse. Medulloblastomas can be divided into 4 molecular subgroups, one of which, the Sonic hedgehog (SHH) subgroup, is further divided into SHH with wild type p53 or mutant p53, which bears the worst prognosis. The SHH subgroup features the most frequent local recurrence. However, there is a limited understanding of mechanisms causing recurrence, and recent studies have shown that genetic drivers of recurrence may be completely different than the original tumor drivers, rendering targeted therapies against the primary tumor drivers pointless. Using mouse models for Shh medulloblastoma and primary cultures of cerebellar granule neuron progenitor (CGNPs) cells, proposed to be Shh medulloblastoma cells-of-origin, we have made the observation that the oncogenic DNA- and RNA-binding protein YB1 promotes DNA repair after radiation. We have also observed that YB1 localizes to the perivascular niche (PVN) tumor cells in mouse and human medulloblastoma. These cells are known for their radiation resistant properties. We have also observed that HIF1a, whose mRNA translation is promoted by YB1, is likewise found in the PVN, where we hypothesize that it promotes maintenance of the stem cell-like phenotype, which also confers radiation resistance. HIF1a is normally degraded under normoxic conditions, however our preliminary studies indicate that it is stabilized in a reactive oxygen species (ROS) and NADPH oxidase (Nox4)-dependent manner. Here, we propose to use mouse Shh medulloblastoma models to test the hypothesis that pharmacologically targeting YB1 in vivo can induce tumor cell death in the perivascular niche, and that in vitro and in vivo treatment of mouse medulloblastoma with inhibitors of ROS or NADPH oxidase will destabilize HIF1a and similarly increase radiation response. Finally, we propose a translational aim wherein we will utilize a novel microfluidic drug delivery platform to test the combined effects of YB1 and ROS inhibition on cell viability in freshly resected human medulloblastoma tumor slices, along with analysis of HIF1a, YB1, and Nox4 localization in primary and recurrent medulloblastoma specimens from a newly established biorepository at Children?s Hospital of Atlanta. Our studies have the potential to validate YB1 activity and HIF1a stabilization as novel therapeutic targets in medulloblastoma, and to set a precedent for screening patient samples to predict response to drugs that could be given in the event of relapse, or to improve radiation response in the primary tumor.